This invention relates to a method that provides a user with the ability to choose the combination of dye species that best fits his or her application. By practicing this invention, users can create prints that are colorful, lightfast, or some combination of these two. In one embodiment, the invention takes into account the characteristics of the printing medium and determines from that information how to optimize the lightfastness and gamut of an image.
The use of inkjet printing systems has grown dramatically in recent years. This growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today""s inkjet printers offer acceptable print quality for many commercial, business, and household applications at costs fully an order of magnitude lower than comparable products available just a few years ago. Notwithstanding their recent success, intensive research and development efforts continue toward improving inkjet print quality. A surge in interest in inkjet printing has resulted in the need to produce high quality prints at a reasonable cost. The challenge remains to further improve the print quality and lightfastness of inkjet prints. The use of large format inkjet prints for point-of-purchase displays, posters and signage, requires high-resolution images to be durable, and to retain their color fidelity for as long as possible.
Color inkjet printers typically use three inks of differing hues: cyan, magenta, and yellow. Black is often added as a fourth ink. The particular set of colorants used to make the inks in the inkjet printer is called a xe2x80x9cprimary dye set.xe2x80x9d The printer is not, however, limited to printing only the colors contained in the primary dye set. A spectrum of secondary or dilute colors can be generated using different combinations of the primary dye set. An inkjet printer using three primary inks and a black ink typically houses these four inks in chambers within the printer cartridge body. In recent years, there has been an increase in the number of inkjet printing devices employing more than four inks in its primary dye set. In particular, some inkjet printing devices have added light or dilute cyan and light or dilute magenta dyes for a total of six inks that improve grain and tonal continuity of printed images. The addition of more inks to the primary dye set can significantly improve improved image quality. Some Hewlett-Packard inkjet printers, for example, employ two cartridges with three chambers each. In these cartridges, the six chambers contain the following inks: cyan, magenta, yellow, black, dilute cyan, and dilute magenta.
As is well known in the art, a successful ink set for color inkjet printing must be compatible with the inkjet cartridge and the printing system. Some of the required properties for the inkjet ink include: good crusting resistance, good stability, proper viscosity, proper surface tension, thermal stability, little color-to-color and/or color-to-black bleed, rapid dry time, no deleterious reaction with the printhead components, high solubility of the dyes in the vehicle, consumer safety, low strike through on plain or coated papers, high color saturation, good dot gain, and suitable color characteristics. When the foregoing properties, or some subset thereof, are present in a particular ink, skilled artisans may refer to the resultant liquid as a stable ink vehicle.
As is well known in the art, any given perceived color can be described using a color space, such as CIELAB, or Munsell. In the Munsell color space, a given color is defined using three terms, Hue (H), Value (V), and Chroma (C). In the CIELAB color space, a color is also defined using three terms L*, a*, and b*. L* defines the lightness of a color, and it ranges from zero (black) to 100 (white). The terms a* and b*, together, primarily define chroma and hue. The term a* ranges from a negative number (green) to a positive number (red). The term b* ranges from a negative number (blue) to a positive number (yellow). Additional terms such as ho (hue angle) and C* (chroma) are used to further describe a given color, wherein:
h0=tanxe2x88x921b*/a*xe2x80x83xe2x80x83Equation 1
C*=(a*2+b*2)xe2x80x83xe2x80x83Equation 2
The L*, a*, and b* values, or the Munsell H, V, and C, values can be used to calculate the volume of space that a specific dye set can produce. The larger that volume is, the more color the dye set is capable of producing. This volume, referred to as color gamut (G), can be calculated in the CIELAB color space according to Equation 3.                     G        =                              ∑                          i              =              1                        6                    ⁢                      xe2x80x83                    ⁢                                    1              /              6                        *                          (                                                L                  white                  *                                -                                  L                  black                  *                                            )                        *                                          (                                                      a                                          *                      2                                                        +                                      b                                          *                      2                                                                      )                            i                        *                                          Cos                ⁡                                  (                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    h                                    )                                            i                                                          Equation        ⁢                  xe2x80x83                ⁢        3            
where i is the following six colors: C, Y, M, R, G, B
In addition to these characteristics, consumers also typically desire printers capable of printing a wide range of colors and of providing images that maintain their color over time. One measure of how well a printed image retains its color over time is lightfastness. Lightfastness is a measure of how colors fade in a printed image when that image is exposed to light. It is advantageous for printer manufacturers to ensure that printouts retain their color quality for as long as possible. Those skilled in the art quantify lightfastness by measuring the percent optical density loss for a particular image. Using various optical density filter deltas, it is possible to monitor color shifts from otherwise monochromatic signals. Data presented using this metric are described in terms of number of years to fail based upon a worst-case percent optical density loss among pure primaries and hue shifts from filter deltas.
A key dilemma in designing inkjet printers is the tradeoff that must be made between simultaneously trying to increase lightfastness and gamut. Dye-based colorants that produce high chroma, and therefore high gamut, trade this brightness for longevity. Brighter dyes simply burn out faster. Conversely, dyes of lower inherent chroma exhibit greater resistance to various fade mechanisms. There is thus a need for a method that would allow a user to create printouts that maximize lightfastness or gamut, or, in the alternative, lie somewhere in the middle of the continuum between maximum lightfastness and maximum gamut.
The present invention is directed toward a method of that provides a user with the ability to choose the combination of dye species that best fits his or her application. Users can create prints that are colorful, lightfast, or some combination of these two. In one embodiment, the invention takes into account the characteristics of the printing medium and determines from that information how to optimize the lightfastness and gamut of an image. Lightfastness and gamut tend to relate inversely, in that the better the lightfastness, the worse the gamut and vice versa. The lightfastness and gamut of an image can be altered by changing the colorants used to print the image. High-chroma inks produce images with high gamut values. Conversely, images printed with low-chroma inks have increased lightfastness. An embodiment of this invention uses the characteristics of the print medium, upon which the image will be printed, to optimize the tradeoffs that exist between lightfastness and gamut. In this embodiment the relative amount of high- and low-chroma ink varies depending upon the characteristics of the print medium.